s plan a distraction that's only intended to delay action by congress. >> should we have a conversation about school security? yes. should we have a conversation about mental illness and the culture of violence? yes. but we can't ignore the common denominator in all of these deadly massacres: access, easy access to killing machines. >> reporter: the fact is, scott, that one-third of america's public schools already have armed security and it's interesting to note an armed deputy sheriff was assigned to columbine high school near denver in 1999 but failed to stop that attack that killed 13. >> pelley: chip, thank you very much. of course, the best thing is to prevent a mass shooting and the united states secret service has spent years studying assassins and school shooters in order to do just that. brian vossekuil and robert fein wrote two studies for the secret service and they told us in an interview for "60 minutes" that gunmen often leave a lot of clues about their intentions. >> strong key to prevention is to encourage those with information to come forward and to report it. and in